This invention relates to processes for removing atmospheric pollutants, more particularly sulfur oxides and nitrogen oxides, from waste gases such as flue gas.
The combustion of a sulfur-containing fossil fuel (e.g., coal or oil) with air yields flue gas containing both sulfur oxides and nitrogen oxides. Quantities depend on the fuel sulfur content and on combustion conditions; typical quantities are about 0.1-0.5% (more usually 0.2-0.3%) by volume of SO.sub.2, trace amounts of SO.sub.3, and about 300-1500 ppm by volume of nitrogen oxides (NO.sub.x) of which about 85-90% is NO, the rest NO.sub.2. A typical flue gas stream also contains some free oxygen (usually about 1 to about 8% by volume) due to the use of excess air; small amounts of fly ash, balance mostly carbon dioxide, water vapor and nitrogen.
Sulfur dioxide has long been recognized as an atmosperic pollutant, and a number of processes for its removal from gases have been proposed. These processes generally will also remove sulfur trioxide when present. More recently, nitrogen oxides have also been recognized as an atmospheric pollutant.
Wet processes for removing sulfur dioxide from flue gas and other waste gases are well known. Such processes have been described for example in U.S. Pat. Nos. 2,134,481 and 2,405,747; in U.S. Pat. Nos. 3,645,671 to Lindsay I. Griffin and myself, and in my U.S. Pat. No. 3,676,059. In the processes described in these patents, an aqueous ammoniacal absorbent solution containing either ammonium hydroxide (U.S. Pat. No. 2,405,747) or ammonium sulfite (the other patents) is used to remove sulfur oxides, and a gas stream containing sulfur dioxide in greater concentration that the original flue gas stream is obtained on regeneration of the scrubber effluent solution. The by-product sulfur dioxide can be converted by known means either to sulfur (e.g., by the Claus process) or to sulfuric acid. Alternatively, the scrubber effluent can be treated with air to form ammonium sulfate as described in U.S. Pat. No. 2,810,627.
Most processes for removing nitrogen oxides from gases are dry processes. Wet processes for nitrogen oxide removal are relatively few in number. It is known, however, that aqueous caustic is an efficient absorbent for removing NO and NO.sub.2 from gases when NO and NO.sub.2 are present in equimolar amounts, but much less efficient when NO is present in large excess; see H. R. L. Streight, Canadian Journal of Chemical Engineering, Volume 36, pages 3-11, Feb. 1958. U.S. Pat. No. 3,654,103 describes removal of sulfur oxides and nitrogen oxides by scrubbing with an aqueous alkaline solution containing sodium or ammonium thiosulfate or a sodium or ammonium polythionate, with electrolytic regeneration of the absorbent solution.